Hydrogenated block copolymers such as polystyrenepoly(ethylene-co-butylene)-polystyrene (SEBS) block copolymers are important thermoplastic elastomers. Unfortunately, the utility at elevated temperatures of these materials is limited by the relatively low glass transition temperature (Tg) of the polystyrene end segments. Typical SEBS block copolymers lose most of their reinforcing ability above 60.degree.-70.degree. C., even though the Tg of the polystyrene segment is about 100.degree. C.
It would be advantageous to increase the Tg of the end segments of SEBS block copolymers to make them more useful at elevated temperatures. Three methods have been proposed to modify SEBS block copolymers to enhance the elastomeric properties of the polymers at elevated temperatures.
In the first method, monomers other than styrene may be employed in the manufacture of ABA block copolymers. This would produce end segments with a higher Tg than that of polystyrene. Among these monomers are 4-tert-butylstyrene, alpha-methylstyrene, and chlorostyrene. Nevertheless, only styrene has gained commercial importance in the manufacture of end segments of ABA type block copolymers and this method has no commercial interest, in part due to process limitations.
Melt blending high Tg homopolymers with ABA block copolymers has also been used as a method to increase the elastomeric properties at elevated temperatures. The choice of these homopolymers is limited to those which are compatible with end segments and which are incompatible with the mid segment. Some improved properties have been noted at elevated temperatures, but concomitant decreases in other properties caused by the melt blend has prevented this method from major acceptance.
Finally, chemical modification of the end segments of the SEBS block copolymers is the third method which might be used to enhance the elastomeric properties of SEBS block copolymers at elevated temperatures. At the present time, no successful system has been proposed which accomplishes this goal.
Chemical reactions of SEBS copolymers may also cause undesired side reactions such as main chain scission or gelation. Since the mechanical properties of SEBS block copolymers change with molecular weight and molecular weight distribution, it would also be desirable to modify the SEBS copolymers without significantly altering the molecular weight and the molecular weight distribution of the compounds.
At the present time, all of these proposed methods have met with failure. Specifically, when polystyrene is replaced with poly(4-tert-butylstyrene) segments of an equivalent molecular weight, the strength to break this block copolymer will be lower, even though the replacement portion has a Tg of 134.degree. C. compared to 100.degree. C. for polystyrene. This difference in strength is believed to be due to increased compatibility with the mid block segments. Melt blending of course adds other properties of the added materials, and therefore, increases in Tg results are sometimes offset by decreases in other properties. Otherwise, the high Tg homopolymer would be used alone.
U.S. Pat. No. 4,868,245, Pottick et al, acknowledges that materials such as SEBS have poor high temperature strength, particularly above the Tg of 100.degree. C. Pottick et al focuses on increasing the Tg of the arene A block. Another attempt is shown in U.S. Pat. No. 4,946,899, Kennedy et al, by direct reaction during polymerization.
At the present time, no olefin based thermoplastic elastomer has been chemically modified to withstand the higher temperatures for extended periods. This is desirable, particularly if the polymers would be useful above at least 100.degree.. Otherwise, the materials are not able to be employed in medical applications where sterilization is important. Automotive and adhesive applications where strength at high temperatures is important are also not possible.
It is therefore an object of this invention to provide SEBS block copolymers which are useful above 100.degree. C.